Scilab – numerical computation for engineers and scientists
scilab.org
scilab.org
In other words, it’s straight forward to do many Signal Processing tasks in Matlab, but it was really difficult (or almost impossible) how to figure out how to perform those same tasks in Scilab.
Suggestion: take popular course book(s) of Signal Processing with Matlab and reimplement all the solutions in Scilab.
Source: interned there sometime along the way, met some great people.